Leakage minimizing shield for electron discharge devices



Feb. 14, 1950 2,497,660

6. F. DEVlNE LEAKAGE MINIMIZING SHIELD FOR ELECTRON DISCHARGE DEVICES Filed June 18, 1945 Inventor-z George F. Devine,

His Attorne Patented Feb. 14, 1950 LEAKAGE MINIMIZING SHIELD FOB ELECTRON DISCHARGE DEVICES George F. Devine, Easton, Conn.,assignor to General Electric Company, a corporation of New York Application June 18, 1945, Serial No. 600,009

Claims. (01. 250-141) This invention relates to electron discharge devices such as cathode ray tubes and particularly to apparatus for minimizing the leakage of current from such devices to the supporting structures thereof.

Cathode ray tubes commonly comprise an envelope within which is arranged an electron gun for producing a beam of electrons, together with an accelerating electrode and afluorescent screen on which the beam is caused to impinge and produce a luminous spot the intensity of which may be varied in accordance with a signal introduced on a control grid in the path of the beam. De-

flecting electrodes are also provided to determine the point on the screen where the beam falls. During the operation of cathode ray tubes the luminescent screen attains a high positive potential with respect to the cathode due to secondary emission. The supporting structure for a cathode ray tube frequently comprises a frame or collar arranged about the envelope of the tube peripherally of the luminescent screen, and there is a tendency for current to leak through the envelope to the supporting structure. The leakage is negligible in dry weather but during conditions of high humidity in the atmosphere the rate of leakage may become appreciable and as a result the signal portrayed on the luminescent screen is affected thereby. For example, the length of a horizontal trace on a cathode ray screen will change with the intensity of the signal so that if a cathode ray tube is provided to give a calibrated reading this reading will be in error when the atmospheric conditions change. Accordingly it is an object of this invention to provide an electron discharge device including an improved arrangement for minimizing the leakage of current from a high potential area of the interior surface of the envelope to the exterior.

It is another object of this invention to provide an electron discharge device such as a cathode ray tube including an improved arrangement for minimizing leakage from the fluorescent screen through the envelope and which is of simple construction and readily applied to cathode ray tubes.

The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization tion taken in connection withthe accompanyingdrawing in which Fig. l is a side elevation, partly in section, of a portion of a cathode ray tube embodying the invention; Fig. 2 is a perspective view, partly broken away, of the external high voltage element shown in Fig. 1; Fig. 3 is a diagrammatic view of the cathode ray tube of Fig. 1 and its asso- The cathode ray tube illustrated in Fig. 1 comprises a glass envelope I including a neck por-' tion 2, only a part of which is shown, and a bowl 3. The beam forming electrodes or electron gun (not shown) is arranged in the usual manner in the neck portion 2, and the face of the tube is provided with a fluorescent screen 4 which becomes luminescent when the electron beam' impinges thereon. An accelerating electrode or anode 5 is provided on the inner walls of the bowl; this electrode may be a coating on the walls or a separate sheet metal electrode. A conducting terminal for the electrode 5 is secured on the outside of the tube as indicated at 6. In order .:.to support the tube in the structure in which it is I to be employed, a resilient collar 1 of rubber or other suitable material is placed about the tube at its greatest diameter around the periphery of the screen, and a viewing plate 8 is secured over The tube is connected in a suitable circuit as indicated in Fig. 3. A suitable high voltage source In shunted by a resistor or potentiometer H is connected to the electrodes as illustrated.

The cathode and first anode shown at l2 and I3,

respectively, are connected to points of the potentiometer providing the required bias voltages; and a control grid shown at I4 is connected to a source of signals It to vary the intensity of the electron beam in accordance with the signal.

The anode 5 is connected through the cap 6 to the high potential positive end of the potentiomi eter. During the operation of the cathode ray tube a high positive potential with respect to the cathode exists on the fluorescent screen 4 due to secondary emission. I This potential may be of the same order of magnitude as the potential of the accelerating anode 5. There is aktendency- 3 for current to leak ofi through the envelope to the supports and surrounding structure, and this tendency is particularly noticeable when there is moisture in the ambient air. The presence of leakage may be detected by changing the intensity of the electron beam; when leakage is present the length of the trace produced-by the sweep of the beam changes when the intensity is changed.

It is to be understood, of course, that suitable deflecting members (not shown) are provided about the neck 2 of the tube to deflect the beam inthe.

desired directions and at the desiredrates. In apparatus in which the trace of the beam of the screen is employed to determinequantities to be measured and in which it is important that'- the scale of presentation remain fixed, the presence of leakage greatly reduces the usefulness of; the apparatus when moisture is present. In order to V minimize leakage and thereby minimize the errors in interpreting the signal, a conducting band' i's provided about the outer wall of the en-- velopead-jacent theperiphery of the luminescent.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In an electron discharge device including an envelope, a luminescent surface positioned on an inner wall of said envelope, a cathode positioned within said envelope, accelerating electrode means located within said envelope between said cathode and said surface, means for impressing on said electrode a potential sufiicient to direct electrons emitted by said cathode onto said surface whereby during operation of said device said surface acquires a high positive potential due to secondary emission and tends to 'produce leakage through said envelope, and -nie'ans -forminimizing leakage through said envelope comprising a conductive band arranged 4 about the outside of; said envelope adjacent the screen. As shown in Figs. 1. and 2, this band'm'ay 4 tential' is of the same order of magnitude as the positive potential on the fluorescent screen 4.

The presence of the same positive potential on the outside of the envelope wall as that within reduces the potential drop to a negligible amount and thereby minimizes the tendency toward leakage of current from the screen 4 to the outside. envelope; The provision of" a conducting band about the tube. in the foregoing manner insures minimum; leakage regardless of changes in atmospheric conditi'orm surrounding the tube and makes it unnecessary tocheck the calibration of the device with changes in such conditions.

'In. some installations the cathode ray tube may be in a relatively exposed position'so that there may be some chance of the operator's touchlmgthe high" potential ring. In such instaliatic-ns-v it is" desirable to provide a resistance of' several megohms between the ring and the anode, terminal 6 as illustrated at l9 in Fig. 4. This re "stance" limits the current which can be drawn. through the ring l6 should the operator come incontact therewith. s

The wire ring It may be embedded in the collar i as illustrated in Fig. 5 where it acts as a rein-forcing member for the collar as well as a high potentialband. for minimizing leakage. Locationof the ring l8 in the collar in this manner avoids-the necessity of replacing the ring each time a new tube-is introduced.

While the invention has been illustrated. in. connection with a cathode ray tube including a ode type having an envelope provided, with.

calibrated screen, other applicationswill readily be apparent to those skilled in the art; forexample, it is obviousthat other, forms of -metal'" bandsincluding coatings onthe envelope may be. employed instead ofa-wire ring. -It is desired, therefore, not to-l-imit-the invention tothe specific construction illustrated and, described,

and-it isintended by the appended claims to cover all -modifications within the spirit and scope of ein e t Q periphery of said surface, and means for maintainin said conductive band substantially at the potential of said electrode means comprising a conductive connection, between said, conductive band-i and said. electrode means;

32.. an electron. discharge device including, an. envelope, means including a cathode and a. first anode for developing an electron beam, 9.

secondvan-o-deufo-r, accelerating said beam, a luminescent screen on an inner wall of said envelope arranged in, the path. of said beam wherebysaid screen acquires a high positive potential due to. secondaryemission, meansincluding an:insulating collar. arranged about said. envelopeadjacent said'screen for supporting said device, a band of con-ducting material. arranged about said envelope. beneath said collar, and means for con- '-='nectin-g said band and said second anode for impressing, on said band a positive potential of.

the same order of; magnitude as that on said screen screen to said supporting. means. I

In; an electronv discharge device of the caththrough the walls of said device to said support is minimized.

4. In combination, an electron discharge de- I vice of the cathode ray typehaving a luminescent screen on an inner wall surface thereof and which attains a high" positive potential during operation of the device due to secondary emission, a conducting member arranged about the 1 outside. of said device adjacent said screen, and means including a high resistance element for.

impressing on, said. member .a, high positive potential-of the same order of magnitude as that on: saidscreen to minimize leakage from said deviceandtominimize the flow of current to said con-ducting, member. z

5.1m combination, an electron discharge device of the cathode ray type having an envelope provided with a luminescent screen'on an inner wall thereof, a supporting means for said device comprising a collar of flexible material arranged about said envelope adjacent the periphery of said screen, a conducting. member embedded in said collar, and means for impr ssing onsaid membera high; positive. potential of the same -;R'+;-'orlderofi magnitude-as. the potentialof said screen.

for minimizing leakage through said produced by secondary emission whereby leakage UNITED STATES PATENTS Name Date Ronci June 7, 1938 Schwartz July 12, 1938 Atwood Mar. 7, 1944 FOREIGN PATENTS Country Date Great Britain Aug. 25, 1937 Australia Apr. 21, 1936 

